This invention relates to photopolymerizable systems which include photopolymerizable compositions that are light-sensitive latices which form water developable printing plates when coated on suitable backing members. Selected image areas can be insolubilized and crosslinked by exposure to actinic radiation and the composition in the non-radiated non-image areas can be washed away with water to provide the desired planographic image.
Emulsion photopolymerizable systems are taught in U.S. Pat. Nos. 4,186,069; 4,224,398; and 4,288,520. In these systems, the photopolymerizable coating is an emulsion mixture of a polymer component having water insoluble polymer particulates within an aqueous suspension medium, together with a water soluble or water dispersible light-sensitive component including a polymer having at least two moieties per molecule. Such polymers are sensitive to actinic radiation, and by generating a free radical or other crosslinking species form a matrix of crosslinked polymer which encapsulate or entrap therein and disperse therethrough the water insoluble polymer. Prior to curing this system is a three-phase emulsion including the water insoluble polymer particulates within the aqueous suspension medium with which is also mixed the light-sensitive polymer. When the coating composition is applied to a suitable backing member, the aqueous suspension is substantially removed to realize a two-phase mixture of the polymer particulates and light-sensitive polymer.
It has long been known to use water insoluble polymers in producing light-sensitive coatings for use on substrates such as planographic printing plates in which the polymers are incorporated as part of a solvent system layer which uses organic materials and which must be driven off from the plate, a requirement which is time and energy consuming, wasteful of solvents and possibly hazardous to the operator and the environment.
Use of water soluble systems generally avoids the disadvantages of the organic base systems, but they are not particularly long lasting because the polymers are water soluble and are therefore subject to erosion of the image area when in use and/or through the action of atmospheric moisture. The emulsion systems of the aforementioned patents provide an improved stability with respect to other water based systems, but they are still characteried by a shelf-life which is less than optimum, especially under high humidity conditions. Additionally, the images provided by such emulsion systems often include spots which are commercially unacceptable.
It is therefore the object of the present invention to provide a photopolymerizable system and a water developable printing plate based on such a system which exhibits improved properties, particularly with respect to shelf-life under high humidity conditions. This and other objects of the invention will become apparent to those skilled in this art from the following detailed description.